The present invention relates generally to a lithography system. More particularly, the present invention relates to improved techniques for inspecting optical projection units, which are used in lithography systems.
Lithography systems used in the manufacture of integrated circuits and flat panel displays have been around for some time. Such systems have proven extremely effective in the precise manufacturing and formation of very small details in the product. In most lithography systems, a circuit image is written on a substrate by transferring a pattern via a beam (e.g., light beam). As is generally well known, lithography systems typically include an illumination unit for transmitting the beam through patterns resident on the surface of a mask and a projection unit for projecting the transmitted beam onto the surface of the substrate.
The projection unit generally contains an optical subsystem having a plurality of optical components that work together to collect and project the transmitted beam. By way of example, the optical subsystem may include optical components such as lenses, prisms, mirrors and the like. Unfortunately, the optical components, either separately or together, may contain imperfections that cause differences between the pattern on the mask and the projected image of the pattern, i.e., the projected image may not coincide exactly with the pattern on the mask. By way of example, the imperfections may be related to mis-aligned optical components or optical components with defects or variations. As should be appreciated, any differences created between the pattern on the mask and the projected image of the pattern make it difficult to ensure precise manufacturing of the product, i.e., the image differs from what is sought and therefore the printed pattern on the substrate is adversely effected. By way of example, the width of printed lines may be increased or decreased, the position of the lines may be skewed or shifted, subsequently processed patterns in the product may be misaligned and the like.
In general, the optical subsystems are certified in the factory before final shipment to the customer. The certification process generally includes testing and adjusting the optical subsystems until they meet desired specifications. Unfortunately, however, due to the inherent high-precision nature of the optical components, testing and adjustments thereto can be difficult to achieve in a cost effective, accurate and speedy manner.
Conventionally, the projection unit has been installed in the lithography system in order to perform testing on the optical subsystems. In most cases, the optical subsystems are tested by exposing photoresist with a projected image of a test mask and then examining the resultant printed image of the projected image in the photoresist. Unfortunately, photoresist exposure and subsequent measurement is typically very slow, and limited in the type of measurements that may be performed, as well as in the accuracy of the measurements. For example, the determination of focus and therefore the focal plane is typically not very accurate in photoresist. Furthermore, there is generally not enough space to perform the adjustments on the optical subsystems when the projection unit is disposed in the lithography system and therefore the projection unit is typically removed from the lithography system when adjustments on the optical subsystems are needed. In most cases, several iterations of testing and adjustments are needed to meet specifications and thus the process of installing and removing is unfortunately slow and time consuming. Moreover, multiple installations and removals may lead to other imperfections of the optical subsystem, i.e., misalignments may be produced when the projection unit is installed or removed.
Thus, there is a need for improved techniques for testing and adjusting the optical subsystem of a projection unit.